US7513873B2 - Low-noise ultrasound method and beamformer system for doppler processing - Google Patents
Low-noise ultrasound method and beamformer system for doppler processing Download PDFInfo
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- US7513873B2 US7513873B2 US11/243,775 US24377505A US7513873B2 US 7513873 B2 US7513873 B2 US 7513873B2 US 24377505 A US24377505 A US 24377505A US 7513873 B2 US7513873 B2 US 7513873B2
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
- A61B8/06—Measuring blood flow
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
- G01S15/88—Sonar systems specially adapted for specific applications
- G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
- G01S15/8906—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
- G01S15/8979—Combined Doppler and pulse-echo imaging systems
Definitions
- This invention relates to coherent ultrasound imaging systems and, more particularly, to phased array ultrasound imaging systems operating in different scan formats and imaging modes. Specifically, but not limited to, the invention relates to phased array beamformer system with low-noise Doppler data acquisition.
- Medical ultrasound imaging systems are capable of many different modes of operation.
- One of these is the Doppler mode dedicated to displaying the movement of blood within a vein or an artery.
- Doppler imaging can be performed using either continuous wave (CW) or pulse wave (PW) techniques.
- CW Doppler acquisition the ultrasound transmitter continuously insonifies the body, while the receiver continuously receives echoes from all objects within the receiver's area of sensitivity. In this case, information received from any specific range interval cannot be isolated. Accordingly, the observation region is the overlap portion between the transmitting and receiving transducer beam profiles.
- the instrument's area of sensitivity is adjusted, by either physical placement of the probe, by beamforming, or both.
- the scatterer As a single scatterer passes the observation region, the scatterer generates a burst of oscillations that contributes to the received radio frequency (RF) signal.
- the frequency of this oscillation is different from the transmit frequency because of the Doppler shift, which is proportional to the component of the blood velocity along the phase gradient of the combined transmitter and receiver beams.
- the “sign,” or relative polarity, of the frequency difference between the transmitted and received signals determines the direction of the blood flow.
- the scanner transmits a periodic pulse wave at a certain operating frequency F 0 that is directed to a particular location having blood flow.
- the signal reflecting from the moving blood is shifted in frequency by an amount proportional to the velocity of the blood flow.
- the received signal has the same essential properties as for the CW.
- the range gate limits the observation region along the beam to the range cell. This allows one to obtain only samples of the Doppler signal with the pulse repetition frequency, the PRF, which introduces the problem of frequency aliasing.
- PW Doppler has limited ability to measure very high blood flow velocities. The rate limitations are fundamental because the transmitted pulse must reach the target and echoes are reflected back to the receiver before the next pulse can be sent.
- CW Doppler transmits a constant continuous wave signal toward the area to be imaged at a particular transducer operating frequency.
- the signal is continuously reflected by the blood flow and received by a receiver.
- the receiver distinguishes between the transmitted signal and the received signal by determining if there is a frequency shift between the transmitted and received signals.
- the movement of the blood causes this frequency shift, where its value is proportional to the velocity of the blood.
- the direction of the blood flow is dependent on whether the frequency of the received signal is greater or less than the frequency of the transmitted signal. Because the signal is transmitted continuously, CW Doppler can detect significantly higher frequency shifts than PW Doppler since there is no inherent sampling rate limitation.
- the separate analog-processing path for a CW Doppler receiver consists of cascaded stages of mixers and filters.
- the hardware includes a number of programmable filters that are tuned to the operating frequencies of the available transducers.
- Such architecture requires using expensive switches and precision components.
- phased array i.e., multi-channel ultrasound systems, it causes a substantial increase in the component count and cost that makes this approach impractical.
- Fazioly et al., U.S. Pat. No. 6,527,722
- a CW Doppler single channel receiver consisting of a mixer accompanied by a bandpass filter (BPF), which operates to translate the RF input signal to a constant intermediate frequency (IF) signal. Consequently, the cost of the CW Doppler processing circuitry will be reduced with respect to a conventional processing system.
- BPF bandpass filter
- Fazioly does not disclose any aspect of CW Doppler data acquisition with a phased-array transducer.
- FIG. 1 depicts a block diagram of the beamformer comprising a plurality of receive channels 110 .
- Each of the channels includes a low-noise amplifier (LNA), a gated quadrature mixer, and a complex rotator.
- LNA low-noise amplifier
- the RF signal amplified by LNA 111 is mixed in a quadrature mixer with a pair of clocks being out of phase by 90° with respect to each other.
- the in-phase clock signal LO I which is supplied to mixer 102 , is provided in common to the in-phase mixers of all of the analog receive channels 110 , as is the quadrature clock signal LO Q received by mixer 104 .
- the outputs of the mixers 102 and 104 are in-phase and quadrature-phase components of a complex baseband signal related to respective RF echo. These outputs are coupled to a complex rotator 106 , which is a baseband signal processing block, that weights, selects, and sums the in-phase and quadrature-phase components.
- the I/Q outputs of the rotator are programmed to represent eight possible phases of the input complex signal.
- the rotator in each channel has its own set of three phase control input bits.
- the in-phase (I) signals 108 of all of the individual Doppler receive beamformer channels 110 are summed in four groups.
- the per-group signals 108 are applied to respective summers 112 having a low-pass pole 114 , which filters out the RF products of the mixing process without affecting the baseband component.
- the partial sums 118 are combined by a summer 116 to generate a beamformed in-phase signal 120 from all channels. It will be understood that the quadrature signals are combined in the same manner.
- the outputs of the I/Q summers are coupled to a downstream processor 140 .
- the processor comprises in-phase and quadrature sections but since they are identical, only the in-phase section is shown. It includes an integrator 122 to integrate (PW) or to smooth (CW) the beamformed signals, a track-and-hold circuit 124 , a high-pass filter 126 to remove clutter signals, an anti-aliasing filter 128 , and an ADC 130 to convert the relatively clutter-free signals to digital format.
- the per-cannel quadrature mixers, 102 and 104 comprise two transistor pairs switching at the LO frequency. While switching, the gates (bases) of the pair exhibits charge fluctuations. Having a spectral density proportional to 1/f, these fluctuations are transferred to the output by multiplication with a time-varying transconductance of the switching pair. Since transconductance of the pair is varied at the 2 ⁇ LO frequency, it contains only even-order harmonics of the LO. This means that flicker noise from the switching pair will directly appear at the output around DC, i.e., in baseband.
- phase noise (Sometimes this noise is referred as the phase noise.)
- the resulting 1/f noise from switching is increased for a factor of N 1/2 as compared with a single channel.
- the beamformer signal gain is equal to N
- the signal-to-noise ratio (SNR) is improved by a factor of N 1/2 .
- the complex rotator 106 sums the weighted baseband outputs of the mixers 102 and 104 . It is followed by the combining of all of the per-channel I/Q output signals represented in the baseband. The noise-referred details of the summing operation are discussed below.
- the LNA/mixer combination needs to provide a gain, which is sufficient to prevent substantial degradation of the SNR by the noise introduced by subsequent summing means.
- a weak signal representing blood flow is situated on a top of a high-amplitude clutter, which may be in the range of 500 millivolts peak-to-peak. Consequently, the entire signal-processing chain needs to be relatively high-voltage in order to avoid signal clipping.
- the latest integration technology is based on low-voltage MOS processes with signal swing of 1.8 Volts or less.
- the LNA/mixer gain may not exceed 12 dB.
- the expected SNR degradation due to 1/f noise can be found as follows:
- f C denote flicker noise corner frequency, i.e., the frequency at which 1/f noise exceeds thermal noise.
- MOS devices manifest a corner frequency, which varies as the reciprocal of the channel length.
- f C has a range of 100 kHz to 1 MHz.
- noise contribution from the LNA/mixer section is primarily related to translating LNA noise from the RF range (2 to 8 MHz, typically) to the baseband. Since the above RF range is well above the 1/f noise corner, the corresponding noise is representative of thermal noise. This noise manifests a noise floor for a subsequent stage, i.e., the summer 112 .
- the noise-floor spectrum introduces by the LNA/mixer section is relatively flat with power spectral density of G 2 ⁇ S T . Because noise contributions from the switching pair and said noise floor are mutually independent, their influence can be considered separately.
- the resulting noise exhibits a linear combination of the above-mentioned noise floor and the input referred noise produced by the summer itself.
- the total noise power can be expressed as the sum of three definite integrals, each related to respective noise source.
- the clutter filter removes any Doppler along with noise components occurring at or near 0 Hertz.
- f MIN denote a minimal frequency of a signal passing the clutter filter.
- the highest Doppler shift determines a cut-off frequency of the processing, f MAX .
- f MAX 100 kHz.
- V N 2 ⁇ f MIN f MAX ⁇ S T ⁇ ⁇ d f + G 2 ⁇ ⁇ f MIN f MAX ⁇ S T ⁇ ⁇ d + ⁇ f MIN f MAX ⁇ S F ⁇ ⁇ d f Evaluating the integrals,
- V N 2 S T ⁇ [ ( 1 + G 2 ) ⁇ ( f MAX - f MIN ) + f C ⁇ ln ⁇ ⁇ f MAX f MIN ]
- V NT 2 S T ⁇ (1+ G 2 ) ⁇ ( f MAX ⁇ f MIN ) Taking the ratio of V N to V NT , the SNR degradation due to 1/f noise of the summer can be expressed as:
- V N 2 V NT 2 1 + f C ( f MAX - f MIN ) ⁇ ( 1 + G 2 ) ⁇ ln ⁇ ⁇ f MAX f MIN
- the present invention includes a Doppler beamformation method and a beamformer system.
- the Doppler beamforming method allows one to achieve a wide dynamic range while operating in a low-voltage environment.
- the new Doppler beamformer outperforms the prior art by simplicity, versatility, lower cost, and higher power efficiency, while maintaining programmability for phase rotating.
- each of a plurality of RF signals is translated to an intermediate frequency (IF) by a mixer that modulates the RF input by a local oscillator clock (LO), IF is higher than the corner frequency, f C .
- IF intermediate frequency
- LO local oscillator clock
- the phases of the produced IF signals are aligned by applying the LO clocks having a selectable angle.
- the aligned IF signals are coherently summed.
- the summed IF output is downconverted to the baseband.
- FIG. 1 is a block diagram of the ultrasound Doppler beamformer known in the art.
- FIG. 2 illustrates arrangement of Doppler spectra and power spectrum of 1/f noise for a traditional baseband conversion and the proposed dual-conversion scheme.
- FIG. 3 is a functional diagram showing the relationship between the elements of a dual-conversion CW Doppler beamformer.
- FIG. 4 is a block diagram of an embodiment of a low-noise beamformer system for CW Doppler imaging.
- FIG. 5 is a block diagram of a second embodiment of CW Doppler beamformer.
- receive beamformers apply controllable delays to the transducer signals prior to summing to steer and focus the receive beam.
- CW mode has no resolution along the range direction. However, it allows one to select a target of interest in the azimuth direction. Fundamentally, the related information is contained in the relative phasing of the RF signal across the channels. Accordingly, beamforming can be achieved through phase shifting of the received signals in a circular range of 0° to 360°.
- FIG. 3 is a functional diagram showing the relationship between the elements of a dual-conversion CW Doppler beamformer.
- the proposed beamformer comprises a plurality of N identical Doppler channels 310 , an N-input summer 312 , and a downstream processor 330 .
- Each of the channels 310 comprises a mixer 302 , an IF filter 306 , and a phase rotator 304 .
- Mixers 302 are operative to translate the frequency of the ultrasound echoes, RF 1 -RF N , to an IF. This is done by mixing input RF signal with a local oscillator (LO) clock.
- FIG. 2 a illustrates the spectral contents of such a signal.
- the ideal mixer is a device, which multiplies two input signals. If the inputs are sinusoids with frequencies denoted as f RF and f LO , the ideal mixer outputs two spectrum lines at the intermediate frequencies f RF +f LO and f LO ⁇ f RF .
- the sum and difference frequencies are usually associated with the upper (USB) and lower (LSB) sideband products of the mixing process, respectively.
- the upper and lower sidebands contain equivalent information as shown in FIG. 2 c ; thus, only one needs to be processed further.
- either the USB, or the LSB products can be selected by filter 306 that produced a plurality of the IF signals, IF 1 -IF N .
- Phase rotator 304 provides phasing of the LO clock on the per-channel basis. This allows to align the IF signals. Combining signals that have been aligned, summer 312 provides a beamformed output, IF ⁇ , as shown in FIG. 2 d . Since this combining occurs at an intermediate frequency, which is above the 1/f corner, flicker noise of the summer is virtually omitted.
- Processor 330 comprises two demodulators, 314 and 316 , arranged for quadrature operation. To operate in quadrature, reference clocks of said demodulators, CLK I and CLK Q , are out of phase by 90° with respect to each other. Both clocks are running at the IF rate.
- FIG. 2 e depicts the baseband representation of the IF ⁇ signal when the outputs of both demodulators are summed geometrically.
- Each of the demodulators is followed by two filters connected sequentially.
- the high-pass filters Removing strong clutter signals from surrounding slow-moving tissue, the high-pass filters reduce the dynamic range of the in-phase and quadrature components, thereby, better utilizing the dynamic range of two analog-to-digital converters (ADC) following the CW Doppler beamformer.
- ADC analog-to-digital converters
- the low-pass filters prevent aliasing of signals or noise, which frequencies exceed one-half of the converters' input sampling rates.
- the I/Q outputs of the downstream processor 330 primarily represent those echo signals that were originated by blood flow.
- FIG. 4 depicts a detailed block diagram of an embodiment of a low-noise Doppler beamformer.
- the beamformer comprises a plurality of N identical Doppler channels 410 , an N-input summer 412 , a downstream processor 430 having an input 426 , and a multi-phase clock oscillator 440 .
- the clock oscillator 440 provides a plurality of k phase-shifted LO clocks having their phases evenly spaced within a 360° range.
- Each of the channels 410 comprises a buffer amplifier 408 , a mixer 402 , an IF filter 406 , and a phase-selecting unit 404 .
- amplifier 408 , mixer 402 , and filter 406 are connected sequentially.
- the LO clock applied to the mixer 402 is derived from the unit 404 .
- Input 430 is operative to receive the RF signals.
- the IF outputs, IF 1 -IF N are provided via nodes 436 .
- the phase-selecting unit 404 comprises a k-input multiplexer 452 , a divide-by-2 counter 454 , a clock terminal 434 , a select port 432 , and an output node 438 .
- terminal 434 receives the entire set of k phase-shifted LO clocks.
- Multiplexer 452 selects one of those in response to a predetermined binary value applied to the port 432 . Then, the clocking frequency is divided by 2 in the counter 454 and outputted via the node 438 .
- RF n cos ( ⁇ RF t+ ⁇ n ) denote an RF signal applied to the input 430 of n-th Doppler channel.
- To produce the IF n signal either the upper or the lower sidebands of the MIX n signal will be filtered out.
- the IF n signals can be aligned in phase.
- the multi-phase clock oscillator preferably comprises a k-phase (k/2-stage) twisted ring counter.
- the ring counter provides k clock lines from inverted and non-inverted outputs; these outputs are coupled to the inputs of the multiplexer 452 via the clock terminal 434 .
- the k-phase ring counter is fed via terminal 450 by an external clock at the rate of 2k ⁇ f LO , where f LO is the LO frequency.
- f LO f IF +f RF .
- Combining of the IF signals is provided by a summer 412 having a beamformed output, IF ⁇ , connected to the input 426 of the downstream processor 430 .
- the downstream processor 430 replicates the arrangement of processor 330 .
- the demodulators, 414 and 416 configured for quadrature operation. Accordingly, reference clocks of the demodulators, CLK I and CLK Q , are out of phase by 90° with respect to each other. Both clocks are running at the IF.
- each of the demodulators is followed by a filter chain.
- This chain consists of a high-pass filter 418 ( 420 ) and a low-pass filter 422 ( 424 ).
- the purpose and operation of these filters are similar to those, i.e., 318 , 320 , 322 , and 324 , shown in FIG. 3 .
- FIG. 5 depicts a second embodiment of CW Doppler beamformer allowing one to minimize the circuit sensitivity to variation in component values.
- CW Doppler beamformer in FIG. 5 also comprises a plurality of N identical Doppler channels 510 , an N-input summer 512 , a downstream processor 530 , and a multi-phase clock oscillator 540 . Moreover, said summer, downstream processor, and clock oscillator are duplicates of respective units in FIG. 4 .
- Doppler channel 510 has no IF filters but outputs both sidebands of the mixing process. Instead of per-channel filtering, there is a single IF filter 560 arranged to select either sideband of the beamformed signal MIX ⁇ . This approach avoids the problem of channel identity at the expense of doubling the amplitude range of signals at the channel output.
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Abstract
Description
Evaluating the integrals,
In the absence of flicker noise, the total noise power would be VNT 2, where:
V NT 2 =S T·(1+G 2)·(f MAX −f MIN)
Taking the ratio of VN to VNT, the SNR degradation due to 1/f noise of the summer can be expressed as:
TABLE I | |||
fmin (Hz) |
1 | 10 | 100 | 1000 | ||
γ(dB) | 8.906 | 8.074 | 7.044 | 5.692 | ||
It can be seen that flicker noise associated with subsequent summing stages increases the system noise floor by a factor of 7-8 dB that substantially degrades the performance of beamforming provided in the baseband.
LO n=cos (ωLO t−θ n)
where θn is the phase of the selected clock.
Multiplying the RFn signal with the LOn clock, the products are:
MIX n =RF n ·LO n=½ cos [(ωRF+ωLO)t+Φ n−θn]+½ cos [(ωRF−ωLO)t+Φ n+θn]
To produce the IFn signal, either the upper or the lower sidebands of the MIXn signal will be filtered out. Thus, properly selecting θn, the IFn signals can be aligned in phase.
-
- 1. Implementing a direct-conversion CW Doppler beamformer, the spectrum of the per-channel quadrature components occupies the same frequencies as flicker noise. This overlapping substantially reduces the resulting SNR of D-mode acquisition.
- 2. Translating a received RF signal to an IF, beamforming may occur at the frequency range, which is above of the 1/f corner. Consequently, the proposed technique allows to improve the SNR as compared with prior art.
- 3. Summing multiple baseband signals, DC offsets, induced by mixers, could substantially reduce the range of beamforming linearity or, in the worst case, saturate the back-end stages. Representation of the per-cannel ultrasound echoes at the IF avoids this problem completely.
- 4. The proposed architecture is particularly suitable for low-voltage process technologies that support broadband applications.
- 5. The described embodiments employ phase-rotating at a fixed intermediate frequency, which makes it easier to predict and obtain repeatable performance of the entire system while implement a wide variety of transducers.
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